The present invention relates generally to switching power supplies and, according to some embodiments, to DC to DC converters designed to mitigate one or more of the issues associated with low duty cycle operation.
Voltage regulators and other power supplies, such as direct current (DC) to DC converters, are used to provide stable voltage or current sources for electronic devices and systems. The typical purpose of a voltage regulator is to convert a source voltage, such as the voltage of an alternating current (AC) or DC power source, into the operating DC voltage of an electronic device. Switching voltage regulators, often referred to as “switching regulators,” are a type of DC to DC converter that convert one DC voltage to another DC voltage with high efficiency. A switching regulator generates an output voltage by converting an input DC voltage into a high frequency voltage, and filtering the high frequency voltage to produce the output DC voltage.
Conventional switching regulators typically include a switch for alternately coupling and decoupling an input DC voltage source (which may be unregulated), such as a battery, to a load, such as an integrated circuit. An output filter, typically including an inductor and a capacitor, is coupled between the switch and the load to filter the output of the switch and thus provide the output DC voltage. The switching regulator operates on the principle of storing energy in the inductor during one portion of a cycle and then transferring the stored energy to the capacitor and the load in the next portion of the cycle. The output filter serves to attenuate any ripple to an acceptable value at the output.
DC to DC converters employing Buck topologies (also referred to as Buck converters) convert an input DC voltage to a lower output DC voltage of the same polarity, and are widely used to step down voltages in diverse applications. In many of these applications the magnitude of the step down from the input voltage to the output voltage has continued to increase with each successive generation of products, pushing the limits of the conventional Buck topology. As will be understood by those of skill in the art, the greater the step down, the lower the operating duty cycle of the Buck converter, i.e., the portion of the operating cycle of the converter during which the input voltage is coupled to the load. For example, applications with a 12 VDC input may now need to be stepped down to operating voltages of 0.4 or even 0.25 VDC. This size of a step, along with the typical requirement that the output voltage be brought up in a controlled manner at system start up, results in operating duty cycles lower than 1%.
Low operating duty cycles can be problematic for Buck converters from a noise perspective as the switching noise associated with the rising edge of the conduction interval of the high-side switch does not have time to dampen entirely before the switch is turned off. This commutation noise introduces noise artifacts in the output voltage of the converter, undermining the goal of a smooth, controlled start up. Such artifacts may be unacceptable, for example, in applications in which previously stored system information must be preserved during start up.
One approach could be to introduce a transformer to handle a portion of the step down, thereby allowing the Buck converter to operate with a higher duty cycle. However, in addition to undermining the simplicity of a Buck topology, the introduction of a step-down transformer may unacceptably reduce system performance and efficiency.